How can I remove a carrier lock on my iPhone?

There is one section of this Support document ("If you don't have another SIM card to use") that suggests that you may need to do a reset after a carrier unlocks your iPhone.

How to unlock your iPhone for use with a different carrier - Apple Support

The reset is presumably to make the phone "notice" that the carrier has unlocked it. A reset alone would not get rid of the carrier lock if the carrier had not removed the lock first.

once the carrier has unlocked it from their side and it still shows locked, you need to factory reset the device.

The carrier is not telling you the truth. Contact the carrier again and ask for a senior support person.

Only the carrier can remove the lock.

I don't think that a factory default reset will solve your current issue. It didn't for me, but I was not using T-Mobile. But, no harm in trying.

Since you appear to be planning to sell the phone, you would want to go through a reset anyway.

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

However, to see if the phone is now really unlocked, you will have to set it up again. If the phone is now unlocked, you can go ahead and perform another factory reset.

If the phone is still locked.....then you need get with a senior support person......not the new kid that is just reading things off the screen in front of them.

Если есть возможность зайти с другого устройства,зайди на сайт How to unlock your iPhone for use with a different carrier - Apple Support

там тебе помогут)

Might it be necessary to reset network settings after the carrier unlocks the iPhone?

Factory reset your device to fix it. (erase all content and settings)

If your iPhone is SIM locked, that is something you need to take up with the carrier.

Once a carrier removes the lock on the phone, nothing more than a restart should be required. It is NOT necessary to completely wipe the phone to start over to remove a carrier lock.

Having same issue, mine is with TMobile. On their site, under my account, it shows the phone is unlocked. But on the phone itself it says SIM locked. TMO "help" is telling me to do a factory reset, but apparently that doesn't solve the problem. Difference is $60 or so when I resell it so would like to be able to unlock.

Factory reset will do nothing to remove the carrier lock.

TMO "help" is telling me to do a factory reset, but apparently that doesn't solve the problem

Are you saying that you did perform a factory reset and the phone was still locked?

Haven't tried yet, I was deferring to your posts but maybe I misunderstood you. Trying to elevate the issue at TMO.

YAY, this worked! Odd thing is that TMO told me the phone was unlocked when it was fully paid up, which was a quite a while ago, and I've certainly rebooted the phone since then.

Glad to help and even more glad it worked for you!